Intermittent dipping in write speed over SMB

Hi,

I have just created a new storage server with the following basic specifications:
CPU: 2x E5-2650v1
RAM: 192GB (24x8GB ECC DDR3 1600)
NIC: Intel X520DA-2
HBA: LSI SAS 9200-8e
JBOD: SC847 E16-RJBOD1
HDD: currently 27x 4TB ST4000DM000
FreeNAS: 9.10.2-U1

zpool (will be adding another raidz2 of 9 drives soon):

Code:

~ zpool status -v
  pool: FloppyD
state: ONLINE
  scan: scrub repaired 0 in 18h10m with 0 errors on Sun Jan 29 15:15:16 2017
config:

        NAME                                            STATE     READ...

Intermittent dipping in write speed over SMB